1. Technical Field of the Invention
The present invention relates to an image forming apparatus for use in an electrophotographic copying machine, printer and the like and more particularly to an image forming apparatus which is arranged to prevent dislocations of images being formed by a series of image formations when composite image forming operation is carried out by repeatedly forming images on the same sheet of paper.
2. Brief Description of the Prior Art
Ordinarily, in an image forming apparatus, the timing for turning on register rollers provided for feeding a paper to a transfer section and the timing for starting exposure for an image onto a photoconductive drum are predetermined so as to always maintain a predetermined correlative relation between the front end of the copy paper being transported to a transfer section located around the photoconductive drum and the position of the image to be transferred on the photoconductive drum. Accordingly, accurate composite image formation must be carried out without having any dislocation of images on the transfer sheet even if plural image forming operations are repeatedly done on the same sheet of paper in composite image formation.
In actuality, however, such dislocations occur. Practically, the movement of a paper can not be made precisely as designed due to the scattering conditions such as the time in starting and stopping register rollers, the difference in each machine, the period each machine was used, and as a result, the front end of a paper and the position of an image on the photoconductive drum cannot correlate each other as predetermined.
A composite image forming procedure will be described with reference to FIG. 3 as an example. At the first image formation, a paper is fed from one of the paper feed rollers 4a-4c to transfer section 10 through register rollers 6, however, on and after the second image formation, the paper travels to the register rollers 6 through paper re-feeding path 14 and paper re-feeding rollers 8 as in the case when a paper is fed from the paper feed rollers 4c. Accordingly, traveling path of a paper which is fed from the paper feed rollers 4a and 4b to the register rollers 6 for the first image formation and the traveling path of the paper which is again fed to the register rollers 6 on and after second image formation differs thereby causing considerable difference in angle of the paper getting into the register rollers 6. Consequently, the position of the front end of each paper which, stops on the register rollers is subjected to be dislocated as shown in FIG. 4 (a) and (b), and there occurs dislocation between the image I.sub.1 being formed at the first image formation and the image I.sub.2 which is going to be formed on and after second image formation.
U.S. Pat. No. 4,175,851 discloses a device which is designed to avoid any positional disagreement between the front end of paper at the transfer section and an image on a photoconductive drum. The device includes a time measuring means for measuring an optionally predetermined time after standard signal has emitted for turning on register rollers or for starting exposure For starting exposure or turning on the register rollers by the output of the time measuring means based on the standard signal, it is arranged to correlate the front end of the paper at the transfer section with the position of the image on the photoconductive drum by adjusting the value set in the time measuring means. However, with such mechanism of device, it can not be effectively dealt with dislocations of front end of paper which occur in the first image formation and on and after second image formation. In order to deal with the dislocations, it may be considered to eliminate the temporary suspension of a copy paper at the position of register rollers on and after, second image formation. However, in such case, it is feared that the dislocation may occur in a composite image formation arising from the dislocations of positions of front end of paper at the position of register rollers when first image formation is carried out. The same problems lie in the device which sequentially form composite images by storing a plurality of copy papers in the paper cycling path for paper re-feeding.